It works. no automatic downloads anymore and no "file:///C:/Users/<User>/Downloads/thepdf.pdf" in the URL bar, but the actual URL.

This add-on seems very usefull for me. I'm working with a few web applications where I have to download and print pdf documents from. Only one of these apllications send pdf with application/pdf content, two of them use octet-stream. That's why I was always forced with dialog to choose if I want save or use viewer and there was no reason to omit this dialog (this is well known issue - I use Firefox 60.0 currently). With this add-on I could achieve common solution for all these three web applications - in my solution a viewer is choosen. One side effect is visible that if we have pdf attachments in e-mails they are also open with a viewer now because I have viewer set in firefox's Applications settings for pdf documents. But I'm going to improve this solution with bash script.

Finally working solution for Firefox 57. Still confusing why similar fix not integrated to firefox natively.